1. Field of the Invention
This invention relates to non-return valves of the sort typically employed in the extraction lines of a steam turbine steam system and in particular to a monitor for indicating the presence and/or position of the clapper of the non-return valve.
2. STATE OF THE RELEVANT ART
The present invention has particular utility in connection with the steam lines associated with a steam turbine. Such turbines are generally and conventionally employed for the purpose of rotating large generators for generating electrical power. Problems with such turbine-generators arising out of the induction of water or cool vapor into the steam turbines become more critical as the units age and particularly as they are used, increasingly, for cycling and/or shift operation. Malfunctions of the equipment in the heat cycle can cause such induction to occur at various locations, including the main steam inlet piping, the hot-reheat steam inlet piping, the cold-reheat steam piping, extraction connections, gland steam-sealing system, and turbine drains. Beyond the resulting structural damage and mechanical malfunction caused by the induction of water or cool vapor into the generator, the resulting unscheduled down time of the equipment is a matter of serious concern.
Water induction incidents have become of such concern that the ASME (American Society of Mechanical Engineers) has established a committee on turbine water-damage and plant design recommendations to prevent water damage are contained in ANSI/ASME Standard No. TDP-1-1985. More recently, studies done for the EPRI on actual operating power generating facilities are set forth in a final report prepared and released by EPRI as Report CS-4285, entitled "Detection of Water Induction in Steam Turbines. Phase III; Field Demonstration." These studies emphasize the continuing, critical need for the monitoring of systems used in the environment of steam turbines, to assist in the prevention of the severe problem of water induction.
By far the extraction line, heater, and cold-reheat lines are responsible for the vast majority of the water induction incidents. And one means which has been provided for prevention or at least minimizing the effect of water induction has been to install non-return valves within the steam turbine extraction lines. Such valves permit flow in one direction and prevent or at least inhibit, flow in the opposite direction. The use of such valves is well known and does not form a part of the present invention.
A significant problem associated with the use of non-return valves is that the same may break in the hostile environment of the steam system associated with steam turbines. These valves and other stationary and non-stationary components within the steam lines and vessels can fail without showing any external evidence of failure. For non-return valves, failure modes may include galling of the claper which prevents its closure and oscillation of the clapper severe enough to cause the clapper to break loose and be carried downstream toward the heater. Not only is damage to the valve itself a concern, but if such failure goes undetected for a long period of time, an ultimate result could be a major turbine failure due to a water induction incident.